A field-effect transistor is one of the most widely-used semiconductor elements. A variety of materials are used for field-effect transistors according to their uses. In particular, semiconductor materials including silicon are frequently used.
A field-effect transistor including silicon has characteristics satisfying the needs for a variety of uses. For example, single crystal silicon is used for an integrated circuit or the like which needs to operate at a high speed, whereby the need for high-speed operation is satisfied. Further, amorphous silicon is used for a device which needs a large area, such as a display device, whereby the need for large area can be satisfied.
As described above, silicon is highly versatile and can be used for various purposes. However, in recent years, semiconductor materials have come to be expected to have higher performance as well as versatility. For example, in terms of improving performance of a large-area display device, in order to realize high-speed operation of a switching element, a semiconductor material which facilitates the increase of the area of a display device and shows higher performance than amorphous silicon is needed.
Some metal oxides have semiconductor characteristics, and tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are known, for example. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Documents 1 to 4, Non-Patent Document 1).
As the metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4).
Under such conditions, a technique relating to a field-effect transistor (also referred to as an FET) using an oxide semiconductor has attracted attention. Further, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn—O-based oxide is applicable to a channel layer of a thin film transistor (Non-Patent Documents 5 and 6).
For example, in Patent Document 5, a transparent thin film field-effect transistor using a homologous compound InMO3(ZnO)m (M is In, Fe, Ga, or Al, and m is an integer greater than or equal to 1 and less than 50) is disclosed.
In addition, in Patent Document 6, a field-effect transistor is disclosed in which an amorphous oxide semiconductor which contains In, Ga, and Zn and has an electron carrier concentration less than 1018/cm3 is used. Note that in this patent document, the ratio of In atoms to Ga atoms and Zn atoms in the amorphous oxide semiconductor is represented as In:Ga:Zn=1:1:m (m<6).
Further, in Patent Document 7, a field-effect transistor is disclosed in which an amorphous oxide semiconductor including a microcrystal is used for an active layer.